The ability to form high quality thin films from a spray deposition process is being sought for volume production of such films useful in solar energy conversion devices and in transparent electrically conductive vitreous substrates, among many other applications. In these applications a high quality film generally means a film having only that thickness required to obtain the desired electrical or photovoltaic effect, the film properties being substantially uniform over the entire surface area of the filmed substrate.
Transparent vitreous substrates, such as glass, may be coated with electrically conductive films for use in a variety of applications. The electrically conductive film can be used as a resistor-type element in order to produce a heated surface when electrical current is passed through the film for use in maintaining an ice-free surface, such as an aircraft windshield. Yet another use for a transparent substrate having a continuous coating of electrically conductive material is to provide a surface which is highly reflective for infrared radiation. Such a surface could then be a heat control device, keeping incident infrared radiation from entering an enclosed space or keeping radiated infrared waves from leaving an enclosed space.
In yet another application, transparent electrically conductive films may be formed in a conductive pattern on a transparent substrate for use in electronic display devices. Electronic display devices, such as liquid crystal devices, require that an electrical field be produced over a medium which has visibility characteristics which are alterable by an electrical field. Such devices, however, require that the resulting display be viewable through at least one of the electrically conductive patterns, thus necessitating a transparent electrically conductive pattern. High quality films are required to maintain a high degree of pattern resolution, optical clarity, and freedom from annoying interference patterns.
A further use for high quality thin films is in a photovoltaic cell, and more particularly, a photovoltaic cell forming a cadmium sulfide (CdS)-type material over a transparent electrically conductive substrate, such as tin oxide (SnO.sub.x). Where the photovoltaic cells are arranged in a "backwall" configuration on a transparent vitreous substrate, incident irradiation must pass through the SnO.sub.x and CdS layers in order to reach the heterojunction where the photovoltaic conversion occurs. Thus, minimum thickness films with a high degree of light transmission are required in order to maximize the light which reaches the heterojunction. A high degree of film uniformity is also required in these applications in order to prevent pinhole defects where an overlaying layer may penetrate an underlying layer to produce various deleterious effects.
A major contributor to producing high quality films from a spray deposition process is the capability of uniformly heating the transparent vitreous substrate throughout the film-forming operation to minimize temperature gradients over the substrate surface wherein uniform film-forming characteristics may be maintained. U.S. Pat. No. 4,224,355, issued to Lampkin et al. on Sept. 23, 1980, which disclosure is incorporated herein by reference, discusses method and apparatus in the prior art for use in heating substrates during film-forming operations. The subject U.S. patent further teaches improved techniques for heating a substrate where radiant heaters are selectively energized to produce a radiant energy input distribution adjacent the substrate approximating the heat loss energy distribution during the film-forming process. The method and apparatus described therein produce high quality films of SnO.sub.x and CdS having uniform qualities not theretofore available from spray deposition processes.
However, the method and apparatus taught by U.S. Pat. No. 4,224,355 obtained some operational problems. The apparatus for conveying a transparent vitreous substrate provided a screw conveyor and a series of trolleys, each trolley supporting a substrate for movement through the system. Such trolleys are subject to mechanical problems and intermittent jamming of the conveying system occurred. Further, considerable handling of the substrate was required with the trolley system. In addition, the trolley system introduced a considerable mass of a metallic material about the periphery of the glass which affected the system capability to maintain uniform substrate temperatures.
Further, the temperature control system discussed therein provided thermocouples generally attempting to sense the temperature of the substrate surface. Although operational control could be obtained from the thermocouples, calibration of the system was difficult and changing system conditions from age, spray accumulation and the like made accurate temperature control maintenance difficult.
The disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus are provided for continuously conveying and heating a substrate while spray depositing materials forming thin films on the substrate.